A dipole is working like a whip antenna

[debojitacharjee]

I have built a dipole antenna to receive FM broadcast, but I have noticed that the antenna works properly only when it's kept horizontally. Why is that?

I also didn't find any difference if one arm (that is connected to the outer shield of the coaxial) of the dipole antenna is either connected or disconnected to the negative (ground) antenna connector of the radio. So, it means that only the arm that is connected to the inner copper wire of the coaxial cable, which is also connected with the positive antenna connector of the radio, works for receiving the broadcast. Then why do we need a dipole antenna? It works like a whip antenna.

 

[N1EXA]

234/frequency x 12 = Length of antenna .
How big is this dipole ?
FM Broadcast band would only be around 29 inches on each side !
Look at a standard car antenna !
Is the input of the radio Coax 50 ohm or 2 screws at 300 ohms ?
For HF the Signal is Usually Horizontal - But reflecting off the Atmosphere it can be a mix of Horzontal ot Verticle.
Also Noise as greater on hf on a verticle antenna.
HF Broadcast Stations want to direct thier signal in a certain direction - Cant do that with a Verticle.

Pete N1EXA

 

[lenk911]

For HF the Signal is Usually Horizontal - But reflecting off the Atmosphere it can be a mix of Horzontal ot Verticle

Not true: Regardless of the polarization when transmitted, reflections off an atmospheric layer is from an engineering standpoint considered horizontal. It keeps sanity in the two way bands by introducing a 20 db cross polarization loss in the offended receiver with a vertical antenna system.

HF Broadcast Stations want to direct thier signal in a certain direction - Cant do that with a Verticle

Yes you can. You just need reflectors and possibly a director element in the antenna system. On short wave these are nearby towers like an AM broadcast station has. Although it is not common on the HF band (> 2 MHZ).

 

[prcguy]

Most FM stations use a circular polarized antenna which will be received on a linear polarized vertical or horizontal antenna the same, you usually can’t see any difference moving your linear polarized dipole receive antenna from vertical to slanted to horizontal. There are still some FM stations that use a linear polarized horizontal transmit antenna and that will definitely work better when your linear pol receive antenna is horizontal.

The 20dB difference between vertical and horizontal is not an accurate number, a good linear polarized transmit antenna should give at least 35dB and possibly up to 40dB of cross pol rejection. It’s reflections from terrestrial objects that screw up and degrade the polarization which may be only 6dB by the time it and all the multi path reaches your receive antenna. For space borne or earth to satellite comms you will easily see the 35+dB cross pol rejection because there upis nothing to screw it up.

A dipole is not simply a whip, the element connected to the shield carry’s the same amount of RF current as the one connected to the coax center conductor. Together they make up the dipole element and here is some more detail on how it works.

Dipole antenna - Wikipedia

en.wikipedia.org

 

[debojitacharjee]

Most FM stations use a circular polarized antenna which will be received on a linear polarized vertical or horizontal antenna the same, you usually can’t see any difference moving your linear polarized dipole receive antenna from vertical to slanted to horizontal. There are still some FM stations that use a linear polarized horizontal transmit antenna and that will definitely work better when your linear pol receive antenna is horizontal.

The 20dB difference between vertical and horizontal is not an accurate number, a good linear polarized transmit antenna should give at least 35dB and possibly up to 40dB of cross pol rejection. It’s reflections from terrestrial objects that screw up and degrade the polarization which may be only 6dB by the time it and all the multi path reaches your receive antenna. For space borne or earth to satellite comms you will easily see the 35+dB cross pol rejection because there upis nothing to screw it up.

A dipole is not simply a whip, the element connected to the shield carry’s the same amount of RF current as the one connected to the coax center conductor. Together they make up the dipole element and here is some more detail on how it works.

Dipole antenna - Wikipedia

en.wikipedia.org

That's the point. The center wire of the coaxial cable mainly carries the radio signal to the receiver. So, what is the use of connecting t he outer shield of the coaxial cable to the other arm of the dipole antenna?

 

[debojitacharjee]

234/frequency x 12 = Length of antenna .
How big is this dipole ?
FM Broadcast band would only be around 29 inches on each side !
Look at a standard car antenna !
Is the input of the radio Coax 50 ohm or 2 screws at 300 ohms ?
For HF the Signal is Usually Horizontal - But reflecting off the Atmosphere it can be a mix of Horzontal ot Verticle.
Also Noise as greater on hf on a verticle antenna.
HF Broadcast Stations want to direct thier signal in a certain direction - Cant do that with a Verticle.

Pete N1EXA

I have built a 54 inches (27 inches each arm) long dipole to receive a distant FM station of 106.3 MHz.

 

[paulears]

You've not got the science quite straight. You're talking about multiple different things. If you have an old fashioned SW radio that does FM broadcast, then you only have the 'half' of the system, but remember that antenna losses and gains are measured in dB, and the difference between a proper balanced dipole and half of one, is actually quite small in signal terms. usually we struggle to capture every last dB, but broadcast radio rarely is low strength, the issues are blocking and dead spots. So having it horizontal and swinging around till it's strongest, usually means at 90 degrees rotation, it's the weakest. That is how it works. As people have said, because home listeners might have their antennas vertically or horizontally polarised through intention or ignorance, broadcasters in the FM band tend to use circularly polarised antennas, so your angle of dangle is less critical. At HF, your antennas can be horizontal or vertically polarised, which matters for line of sight or skywave, but if it's bounced off a layer, then that polarisation changes anyway?

For transmitters, only having one half of a dipole matters quite a lot. For the transmitter to work efficiently, the antenna has to be resonant, and balanced or unbalanced. A portable radio only has one half of the antenna - the other is missing, and the match therefore poorer. A dipole is not a dipole without it going both ways - either out to the tips, or with the tips connected to each other - a folded dipole. The key is they must be a mirror image for maximum efficiency. Many bodge antennas might be the wrong length. What happens is that the antenna will either be matched and resonant at the desired frequency, or it won't be. Efficiency drops off and it's just a bit of metal in the sky! A piece of cable with the screen cut, exposing the centre on it's own for a quarter wavelength - like just over 19" for 2 metres works reasonably well. If you were to strip the outer screen covering and fold the screen back down the cable for a 'tube' 19" long, you've sort of made a dipole, but these sleeve dipoles are not that good a match unless you tweak them a bit.

Antenna theory is all over the net - do a bit of googling and you'll get some good ideas and explanations.

 

[prcguy]

That's the point. The center wire of the coaxial cable mainly carries the radio signal to the receiver. So, what is the use of connecting t he outer shield of the coaxial cable to the other arm of the dipole antenna?

The center conductor and the inner side of the shield both carry the signal to the receiver, its two separate conductors working together. The dipole element connected to the shield is just as much of the antenna as the element connected to the center conductor. It’s not much different than trying to use just one wire to connect a speaker to your radio, how is the current going to flow from the + output of the amplifier to the speaker and back to the amplifier - to complete the circuit if you leave off the - wire?

 

[dlwtrunked]

Not true: Regardless of the polarization when transmitted, reflections off an atmospheric layer is from an engineering standpoint considered horizontal. It keeps sanity in the two way bands by introducing a 20 db cross polarization loss in the offended receiver with a vertical antenna system.


Yes you can. You just need reflectors and possibly a director element in the antenna system. On short wave these are nearby towers like an AM broadcast station has. Although it is not common on the HF band (> 2 MHZ).

Actually arriving at the receiver, it may not be "horizontal". The conclusion that it is is based on assumptions that may not be true. See Should I chose a vertical or a horizontal HF antenna?

 

[merlin]

That's the point. The center wire of the coaxial cable mainly carries the radio signal to the receiver. So, what is the use of connecting t he outer shield of the coaxial cable to the other arm of the dipole antenna?

Dipole is a two element thing, each arm acts as a counterpoise for the other.
The center point of coax attachment is about 60 ohms, so a close match with either 50 or 75 ohm coax.
The coax is a feedline and can be a balanced twin lead just as well. often twinlead impedance is upward to 300 ohm but a balun can match that
to a coax which is unbalanced.

 

[debojitacharjee]

The center conductor and the inner side of the shield both carry the signal to the receiver, its two separate conductors working together. The dipole element connected to the shield is just as much of the antenna as the element connected to the center conductor. It’s not much different than trying to use just one wire to connect a speaker to your radio, how is the current going to flow from the + output of the amplifier to the speaker and back to the amplifier - to complete the circuit if you leave off the - wire?

But when I disconnect the outer shield of the coax from the receiver, it still works, and I don't find any difference in audio quality. But in case of a speaker, it won't work at all.

 

[prcguy]

But when I disconnect the outer shield of the coax from the receiver, it still works, and I don't find any difference in audio quality. But in case of a speaker, it won't work at all.

If you measure the signal then remove the grounded dipole element you will see the signal drop in level.

 

[paulears]

You cannot apply DC or even AC science to antennas. You are thinking of circuits, so breaking the circuit fails it, but that is not what happens with RF. losing one half probably means a signal strength drop of between 3 and 6dB in real terms, but with bags of signal, 6dB probably won't increase noise in a broadcast receiver. At 30 miles away, with a poor path between you 6dB could be VERY important.